A newborn is diagnosed with hemolytic disease of the fetus and newborn (HDFN) caused by anti-Kell alloantibodies. Unlike anti-D, anti-Kell causes more severe fetal anemia by a mechanism beyond hemolysis alone. What additional mechanism explains the disproportionate severity?

• A Kell antigen is expressed on erythroid progenitors, suppressing fetal erythropoiesis ✓
• B Anti-Kell crosses placenta more efficiently than anti-D
• C Anti-Kell activates complement more efficiently, causing intravascular hemolysis
• D Kell antigens are expressed on hepatocytes, causing fetal liver dysfunction

Explanation

The Kell (CD238) antigen is expressed on erythroid progenitor cells in addition to mature erythrocytes. Anti-Kell antibodies suppress erythropoiesis by destroying early erythroid progenitors, leading to fetal anemia that is disproportionately severe relative to the degree of hemolysis. This erythropoietic suppression means reticulocyte counts are unexpectedly low ('hyperregenerative' response is absent), and fetal anemia can occur even when amniotic fluid bilirubin levels (which reflect hemolysis) are relatively low. This distinguishes anti-Kell HDFN from anti-D HDFN.

Reference: Robbins & Cotran Pathologic Basis of Disease, 10th ed.

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